Compensation of coaxial cables



- July 1 1969 A. LI EBSCHER COMPENSATION OF COAXIAL CABLES Filed Sept. 8, 1966 M/VENTOI? ARTHUR L/EBSCHER N WE u YQQR bwnsQ ATTORNEY I United States Patent 3,452,429 COMPENSATION OF COAXIAL CABLES Arthur Liebscher, Jenkintown, Pa. Electronics Inc. of Pennsylvania, 2440 Maryland Road, Willow Grove, Pa. 19090) Filed Sept. 8, 1966, Ser. No. 577,919 Int. Cl. G01r 27/26 U.S. Cl. 29593 6 Claims ABSTRACT OF THE DISCLOSURE A method of compensating coaxial cable in which the outer conductor of the cable is squeeze deformed at selected locations to modify the capacitive relation of the outer conductor to the inner conductor and thereby bring the voltage standing wave ratio within predetermined limits.

This invention relates to the compensation of coaxial interconnecting cables of any desired length.

A well known type of coaxial cable includes an inner conductor surrounded by an insulating layer which is in turn surrounded by an outer conductor in the form of a braided cover or tubing which is corrugated or spirally Wound and with an outer insulating and protecting layer. These cables include connectors, and those known as Military Type N and BNC. Various plug type end connections for such cables are employed, these commonly including a plug and jack or a plug and socket, with a pin and socket connection for the inner conductor.

Coaxial cables with their end connectors, dependent upon their use, must have a voltage standing wave ratio, referred to as VSWR, below a predetermined limit for satisfactory performance of such cable.

Various tests have heretofore been applied to determine the VSWR, including the time domain test which sends a pulse down the line. Some of the pulse energy is refiected back and the equipment shows the location of any change in impedance in the line as well as the effect of any correction which may be attempted. This equipment requires interpretation of impedance reading to calculate the voltage standing wave ratio.

Another test heretofore employed was by a conventional slotted line with a single frequency applied at a desired level.

Wide departures in use from the applied frequency tended to render uncertain the results of such a test unless time consuming multiple points are also tested.

Sweep frequency methods are commonly used in production testing, wherein the voltage standing wave ratio or VSWR response of an entire band of frequencies is simultaneously depicted as an oscilloscope pattern. This permits observation of dynamic changes in the VSWR response due to mechanical changes or adjustments in cable or connector construction.

It is the principal object of the present invention to compensate for deviations by simple but effective procedures thereby to provide the best coaxial condition or linear capacity relation between the outer and inner conductors.

It is a further object of the present invention to improve the performance characteristics of coaxial cables in a simple but effective manner with a preliminary determination of the preferred locations for compensation and with a permanent correction applied at such locations.

It is a further object of the present invention to compensate coaxial cables by bringing the voltage standing wave ratio within predetermined limits and retaining the ratio within such limits.

a coaxial cable with well known type of end connections I attached thereto and with compensating elements applied thereto;

FIG. 3 is a transverse sectional view taken approximately on the line 3-3 of FIG. 2; and

FIG. 4 is a drawing of an oscilloscope pattern for a particular uncompensated coaxial cable and with the same cable compensated.

It should, of course be understood that the description and drawings herein are illustrative merely and that various modifications and changes can be made in the structure and methods herein set forth without departing from the spirit of the invention.

Like numerals refer to like parts throughout the several views.

In accordance with a preferred mode of practicing the invention the particular coaxial cable to be compensated has a resistance load applied to one end thereof. A sweep generator is coupled to a wave guide through a first transducer and calibrating means and connected to the cable, with a second transducer interposed between the wave guide and the cable. The wave guide has a directional coupler which picks up the combination of a reflected wave and a forward wave which is detected for observation on an oscilloscope.

Pressure is preliminarily applied to the cable by squeezing the cable around the circumference at locations spaced from the connectors and as determined visually on the oscilloscope for the best reduction of VSWR, and a permanent correction is preferably then applied at the selected locations.

The correction can be applied in any suitable manner to effect a permanent deformation at one or more selected locations, one suitable correction being provided by compressing and permanently deforming continuous bands or rings into engagement with the exterior of the cable.

Referring now more particularly to FIG. 1 of the drawings, a coaxial cable 10 having end connections 11 and 12 has a resistance load 13 connected thereto. The cable 10 may be of the type which includes an inner or central conductor 14 surrounded by an insulating and spacing layer 15 which in turn is surrounded by a conducting layer 16 with a surrounding insulating and protecting layer 17. The conducting layer 16 may be of single or double braided metallic strands, may be of metallic tubing, and may be of metal corrugated or spirally wound.

A sweep generator 20 of any suitable type is provided, having a sweep range determined by the ultimate use to be made of the cable 10. The range may be from megahertz (megacycles) to high gigahertz (gigacycles) of a level which the cable 10 can carry. As an example of a specific range, the frequencies may be from 4 to 12 gigacycles. The sweep generator 20 is connected through a filter or attenuating device 21 and through a transducer 22 to one end of a wave guide 23 to which, at the opposite end, the cable 10 is connected through a transducer 24.

The wave guide 23 preferably includes a directional coupler 25 which is connected in any suitable manner,

such as through a crystal detector 26 to an oscilloscope 27.

If the outer conducting layer 16 is of braided strands or otherwise lacks the capability of retaining a deformation applied thereto, the cable 10, before the attachment thereto of the end connectors 11 and 12 has a plurality of loose continuous rings or bands 30 slid thereon. The bands 30 are of copper, aluminum or any other material of adequate strength and capable of permanent deformation as hereinafter explained.

The manner of compensating a cable will now be pointed out.

With a cable 10 mounted as shown in FIG. 1, the sweep generator 10 is activated to apply the desired frequency range, say 4 to 8 gigacycles, 8 to 12 gigacycles or 4 to 12 gigacycles through the filter 21 and transducer 22, wave guide 23 and transducer 24 to the loaded cable 10.

The directional coupler 25 will pick up a combination of a reflected wave and a forward wave and the trace of this is available, after detection by the detector 26, for observation at the oscilloscope 27.

Referring now to FIG. 4, this figure represents the screen of the oscilloscope 27, with ordinates representative of the voltage standing wave ratio, and the abscissas repreabove it is preferred to correct first for the end of the sentative of the progression of the combination wave. The i curve 32, shown as a broken line, may be taken as a curve showing the condition of the cable 10 as manufactured and with the connectors 11 and 12 thereon. If an acceptable voltage standing wave ratio of the order of 1.2 be taken as the standard, it will be noted that the peaks of the curve 32 extend beyond that ratio value.

The compensation is effected by compressing the cable 10 at one or more locations with any desired implement, such as pliers, and observing the effect of the compressing as shown by the oscilloscope trace. The locations which tend to improve the ratio and flatten out curve 32 are determined by trial and error both as to the distance from the connectors 11 and 12 and as to the extent of this initial compression. The rings are preferably moved to the lengthwise locations on the cable 10 as indicated by the initial compressing and are then compressed by any suitable compressing or collapsing tool. As the compression of the rings 30 is being effected the trace on the oscilloscope 27 is observed and when the desired trace is obtained further compression of the rings 30 is terminated. The curve 33 shows a trace obtained for a particular cable 10 having an initial trace at 32, after compensation.

In determining the best locations of the squeeze points along the cable 10 an elfort is made to reduce all the high points but the locations giving the best reduction are preferably used.

The compression and permanent deformation of the rings 30 at locations to give improved characteristics of the cable 10, results in a capacitance change between the outer and inner conductors 16 and 14.

If the outer conducting layer 16 is a metallic tube, capable of taking and retaining a permanent set, the rings 30 will not ordinarily be required.

In making the compensating corrections as described cable having the worst condition but in testing a family of cables after the first few have been tested it will soon become apparent which end has the worst condition requiring correction.

It will thus be seen that simple but effective apparatus and methods have been provided for attaining the objects of the invention.

I claim:

1. The method of compensating a coaxial cable to an acceptable voltage standing wave ratio which comprises:

resistance loading the cable,

applying a predetermined frequency to the cable and determining the voltage standing wave ratio, and squeeze deforming at a selected location the outer conductor of the cable to change the capacitive relation thereof to the inner conductor and thereby reduce the voltage standing wave ratio.

2. The method as defined in claim 1 in which:

said deformation includes an initial trial deformation.

3. The method as defined in claim 2 in which:

an endless ring is applied to the cable before said frequency is applied, and

said ring is squeezed to provide a permanent squeeze deformation of said outer conductor.

4. The method of compensating a coaxial cable as defined in claim 1 which includes:

transferring the applied frequency through a wave guide having a directional coupler, and

detecting the voltage standing wave ratio and providing a trace of the ratio. 5. The method as defined in claim 1 in which: said deformation includes a permanent deformation, affecting the voltage standing wave ratio in one direction. 7

6. The method as defined in claim 1 in which:

said deformation includes an additional subsequent permanent deformation affecting the voltage standing wave ratio as determined upon reversal of the cable.

References Cited UNITED STATES PATENTS 2,516,528 7/1950 Purcell 29-600 X 3,158,825 11/1964- Vetter 333-83 3,287,672 11/1966 Heinz 333-83 3,349,479 10/1967 Sewell 29600 OTHER REFERENCES An Accurate Substitution Method of Measuring the VSWR of Coaxial Connections, A. E. Sanderson, The Microwave Journal, January 1962.

JOHN F. CAMPBELL, Primary Examiner.

D. C. REILEY, Assistant Examiner.

US. Cl. X.R. 29-600; 33396 

